Alnwick joined Newcastle United in 2008 from Sunderland, and went on to represent England at under-17 and under-18 levels. He spent some time on loan at Gateshead in the 2011–12 season and played 6 Premier League games for Newcastle in the 2014–15 season. He was loaned to Bradford City in March 2015, and Port Vale on a free transfer later that year in August. He was then sold to Rangers FC in January 2017 but spent the 2018–19 season on loan at Scunthorpe. Alnwick joined Blackpool on loan for the 2019–20 season and then moved on to St Mirren on a free transfer in June 2020 and spent two years there, before returning to the English Football League with Cardiff City in May 2022.